Ceiling mounted fans are often used for circulating air within large buildings such as warehouses, factories, gymnasiums, retail stores, auditoriums, convention centers, theaters, or other buildings with large open areas. For fire safety, a matrix of overhead sprinklers are usually installed to quench any fires that might occur within the building.
To detect a fire and control the operation of the fans and sprinklers appropriately, various types of fire sensors are available. They usually operate by optical detection (photoelectric), chemical reaction (ionization), or heat detection (fusible link or infrared sensor for radiation).
Some optical photoelectric smoke detectors comprise an infrared light beam passing at a right angle in front of a photodiode or other photoelectric light sensor. In the absence of smoke, the light beam passes undetected in front of the light sensor. Smoke particles, however, can scatter the light beam into the sensor and trigger the smoke detector.
In other types of optical photoelectric smoke detectors, known as projected beam detectors, an emitter projects a light beam across a room where a distant light receiver senses the intensity of the beam. When smoke disperses the beam, the receiver provides an alarm signal in response to sensing reduced light.
Ionization style smoke detectors emit alpha radiation to create a small electrically conductive ionized path between two electrodes. When smoke absorbs the alpha particles, the smoke disturbs the ionized path and interrupts the current between the electrodes, thereby triggering the detector.
Some fire detectors (e.g., heat detectors) are in the form of a fusible link incorporated within a sprinkler head. The fusible link holds a valve of the sprinkler closed until sufficient heat from the fire melts or otherwise destroys the link, thereby activating the sprinkler.
In many cases, the sprinklers are fed by a pressure vessel containing a limited supply of water that is at a pressure higher than that of the municipal water that fills the pressure vessel. This allows an individual sprinkler or a group of sprinklers in a single zone of a multi-zone system to rapidly and intensely focus high-pressure water at a localized area before the fire has time to spread.
If the location of the fire is not accurately determined and, as a result, the wrong sprinklers are activated, this can waste the high-pressure water on an area that does not need it. Depleting the limited supply of high-pressure water in this manner might allow the fire to spread with only lower pressure water, if any, left to suppress it.